Particles having a hollow in their inside (having a hollow surrounded with a shell) are called hollow particles, and are used as, for example, an optical scattering material, a heat insulating material, a low reflection material or the like. The particles are also used as microcapsule particles by encapsulating various substances in their hollow. For example, Japanese Unexamined Patent Application, First Publication No. 2001-233611 (Patent Document 1) and Japanese Unexamined Patent Application, First Publication No. 2007-070458 (Patent Document 2) describe hollow silica particles consisting of a silica shell and a heat insulating paint using the same.
As a method for producing hollow particles, for example, Japanese Unexamined Patent Application, First Publication No. 2002-80503 (Patent Document 3) and Japanese Unexamined Patent Application, First Publication No. 2005-215315 (Patent Document 4) describe a method for producing hollow particles by preparing oil droplets comprising a radical polymerization reactive monomer, and a hardly water-soluble organic solvent having low compatibility with a polymer of this monomer in a water solvent, followed by polymerization.
Japanese Unexamined Patent Application. First Publication No. 2006-89648 (Patent Document 5) also describes a method for producing hollow particles by preparing oil droplets containing a reactive monomer having a radical polymerizable functional group and a crosslinkable functional group, and an organic solvent, and after polymerization, reacting the crosslinkable functional group.
Moreover, International Publication No. WO 2005/097870 (Patent Document 6) describes hollow particles obtained by emulsifying, in a polar solvent, a reaction solution consisting of a reactive silane coupling agent, a non-reactive solvent, and a polymerization initiator, and polymerizing, and hollow particles obtained by emulsifying a mixed solution consisting of an epoxy prepolymer and a non-reactive solvent in a polar solvent, adding a polyamine to prepare hollow resin particles, and inorganic-crosslinking the prepared hollow resin particles with a silane coupling agent having an amino group.
Japanese Unexamined Patent Application. First Publication No. Sho 62-127336 (Patent Document 7) and International Publication No. WO 2005/071014 (Patent Document 8) also describe a method for producing hollow particles obtained by absorbing a crosslinkable monomer in seed particles, and polymerizing the crosslinkable monomer.